1. Field of the Invention
The present invention relates to a hydro-mechanical transmission having both mechanical and hydrostatic power branches and in particular to such a transmission for use in an agricultural tractor.
2. Description of Related Art
Hydro-mechanical transmissions are transmissions that combine a mechanical transmission with a hydrostatic unit. Although mechanical transmissions are generally more efficient and reliable than pure hydrostatic transmissions, they have the disadvantage of not being infinitely variable, as are the more expensive hydrostatic transmissions. Likewise, the hydrostatic transmissions have a major disadvantage in that they are less efficient then mechanical transmissions. Hydrostatic transmissions also generally require larger components, such as larger pumps and motors, as the transmission increases in capacity.
In order to satisfy space limitations, reduce cost, increase efficiency and provide an infinitely variable speed, hydro-mechanical transmissions have been developed that combine the best features of both transmissions. Hydro-mechanical transmissions are typically of a split power input type, where a hydrostatic unit and a mechanical transmission are driven in parallel by the vehicle engine. The hydrostatic output power is combined in the mechanical transmission with the split mechanical power input from the engine to produce hydro-mechanical output power in multiple power ranges. In each range, varying the stroke of the hydrostatic unit can infinitely vary the speed and torque.
While hydro-mechanical transmissions are known in the art, most transmissions suffer from high complexity. Many hydro-mechanical transmissions also require additional gear sets to provide a reverse range.
It is an object of the present invention to provide a hydro-mechanical transmission that is uniquely designed for optimal operation in an agricultural tractor. An agricultural tractor is most often operated in the field at a field use speed or on the road at a transport speed. The field use speed is in the range of 7-12 kph while the transport speed is in the range of 36-60 kph. It is thus desirable to provide a transmission that has its peak efficiencies at these two most commonly used speeds. A hydro-mechanical transmission typically has an efficiency peak in each transmission shift range at the speed where the hydrostatic unit output is near or at zero. At these points, the transmission is operating most like a mechanical transmission and at the higher efficiency of a mechanical transmission.
Hydro-mechanical transmissions typically have one or more shift points where the transmission shifts from one range to another. It is a further objective of the present invention to configure the transmission such that the shift point or points occur outside of the field use speed or the transport speed to minimize shift occurrence. By placing a single shift point between the field use speed and the transport speed, the shift point is only encountered when the tractor is accelerating or decelerating through the shift point. It is unlikely that the tractor will be operated continuously at or near the shift point speed.
It is a further advantage of the present invention to provide a transmission having a physical package similar to existing mechanical transmissions in length, enabling it to be readily integrated into existing tractor designs with minimal modification to the tractor. Many prior hydro-mechanical transmissions do not integrate a reverse gear set into the combining mechanical transmission but instead have a separate gear set before or after the combining mechanical transmission for shifting between forward and reverse directions. Such a design takes more space and may require the vehicle be brought to a stop before shifting between forward and reverse.
It is a still further objective of the present invention to provide a transmission in which the speed can be commanded anywhere in the full speed range of approximately xe2x88x9220 kph to 60 kph without requiring the operator to perform a shift when the vehicle transitions from a reverse direction to a forward direction and vice versa.
The hydro-mechanical transmission of the present invention achieves high efficiency in the field use and transport speeds by selecting the gear ratios to achieve pure mechanical drive at the field use and transport speeds. The gear ratios are selected so that there is no or little hydrostatic power being transferred at these speeds.
The placement of the shift modes outside of the two most common operating speeds is accomplished by providing the transmission with two forward speed ranges, a low speed range and a high speed range, with a single shift point between the two ranges. This coordinates well with the desire for two points of maximum efficiency. The shift point occurs when the variable displacement portion of the hydrostatic unit is at the maximum stroke angle. This corresponds with the point of least efficiency. By locating the point of least efficiency of the transmission at an intermediate point between the field use speed and the transport speed, the amount of time the vehicle is operated at the least efficient speed is minimized. Thus, the transmission efficiency is optimized for the intended agricultural tractor duty cycle.
Another benefit of a transmission having only two forward speed ranges and a single shift point therebetween is that the transmission efficiency curve is flatter at the field use speed and the transport speed. This provides increased flexibility in the speed selection in the field and on the road while still maintaining a relatively high efficiency. This is in contrast to transmissions having three or more shift ranges and shift points where the region of maximum efficiency is in a narrower band between each shift point.
The hydro-mechanical transmission has a planetary system including three planetary gear sets including a reversing planetary gear set together with two clutches and a reverse brake to provide an infinitely adjustable speed change over the speed range from xe2x88x9220 kph to 60 kph without an additional direction change gear set.
The compact package size of the transmission is facilitated by a number of features in the transmission. The hydraulic pump and motor are both located in the same general location along the axis of the transmission, thus minimizing the axial length of the transmission required for the pump and motor. To further reduce the size of the transmission, many components in the transmission are utilized for multiple functions to minimize the number of clutches and other components in the transmission. For example, the combining mechanical transmission uses the sun gear S1 as the fixed mechanical power input during all modes of operation. Likewise, the planetary ring gear 2 is the hydrostatic power input for all operating modes. A single output member is also used in all operating modes. Two clutches are provided, a low range clutch, a high range clutch, and a single reverse brake. In the low range mode, the planet gear carrier is coupled to the output by the low range clutch. In the high range mode, a sun gear S2 is coupled to the output by the high range clutch. In reverse, a reverse brake is engaged to ground a ring gear R3. This drives the output, a sun gear S3 in the reverse direction.